Thinking of making my own PSU

Discussion in 'General Technical Questions and Answers' started by KE0CPH, Nov 7, 2016.

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  1. KE0CPH

    KE0CPH Ham Member QRZ Page

    So I've got a computer power supply and I've been thinking about modifying it a little bit to power my 2 meter radio for APRS. I know the thing supplies 12 volts but so do car batteries when the car is off. I'm only going to push 25 watts out of it anyway (It's a Kenwood TM-281a so my only options are 25 and 65 watts).

    I'm thinking that what I'll need to do is solder all the yellow cables together for a big +12 volt line, solder all the black cables together for ground, then solder all the red cables together for +5 volts to power my Raspberry Pi.

    I then plan on buying a volt and ammeter (maybe a combo device?) so I can make sure I'm getting proper voltage and checking on how many amps I'm drawing.

    Any suggestions? Should I not do this, do it, anything?

  2. K7MH

    K7MH Ham Member QRZ Page

    There are articles about converting them to bench supplies if you Google converting a computer power supply.
    I have done it before but not for a ham radio related power supply. I would not tie all the wires together. I would rather have
    one set for each voltage used that is the right wire size. As I recall you need a 10 watt resistor to create a load so it will turn on.
    I would read up on it before getting started.
  3. K8ERV

    K8ERV QRZ Member QRZ Page

    As said, you may need to put a load resistor on the 5v wire to get the PSU to turn on. And ATX supplies may not go much higher than 12 volts, could be bit low for a ham rig. Some may have an adjusting pot inside that will boost it a bit.

    I don't know about possible RFI, I don't have that situation. Good luck.

    TOM K8ERV Montrose Colo
  4. KB7QPS

    KB7QPS Ham Member QRZ Page

    Car batteries provide 13.8V, not 12V, with the car off and the battery charged.
  5. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    What's the brand and model of the power supply?

    Some handle rapidly varying loads (such as CW and SSB) very well and some don't handle them so well, and don't really need to since a personal computer doesn't need to switch from 0.1A to 25A in milliseconds, and do that repeatedly.

    However if you're going to use it only on 2m FM and that's the only application, that's a fairly easy one as the current demand is steady state once you key down -- so if the PSU takes some milliseconds to settle, it probably doesn't matter as long as it doesn't overshoot.
  6. GM1FLQ

    GM1FLQ Ham Member QRZ Page

    Top tip.......

    Not on 2 metres preferably , better to park the push/pushin on 27mhz. ;)

    Just something for you to think about - for probably less than the cost of your meters you could probably buy a better SMPS that would comfortably deliver enough current for you to run your rig at 65w and be voltage adjustable to 13.8v and have some form of protection circuitry.

    You can find a selection on eBay.
    Last edited: Nov 7, 2016
  7. KB0MNM

    KB0MNM Ham Member QRZ Page

    If you want an excercise in frustration, try to use that computer switching supply without a few modifications. The older ones often required a certain wire ( white in some cases, pun intended ) switch high or low after a slight delay ( consult the original IBM AT documentation for details ). As others point out, you really want a 13.2 to 13.8 VDC filtered supply, because the alternator system in your car ( for your mobile rig ) puts out voltages in that range. The switch-mode supply from a computer may or may not rely on the filter capacitors on various peripherals (eg. floppy disk drive, hard drive, etc. ) to smooth out 'ripple'. Most of these supplies also will not start if there is an insufficient load, which is why folks suggest a big resistor. Yet folks have done it before, just consider using a 'dummy load' of the DC sort ( not a Cantenna ) to check the output with an oscilloscope before connecting it to your rig. The start-up can be an issue in certain cases, and could be more trouble than what it is worth. A used linear supply would probably save you money in the short term, or get a newer switch-mode supply from a reputable source for long-term savings. MCM amd MFJ would have some of the less expensive options*. Remember that 65 watts RF output is more than 10 amps, but 65 Watts DC input is not. W=ExI ( P=E * I ) for DC is not the same for alternating current output. *There are plenty of suppliers, and the used equipment forum here at QRZ could save you even more- if the shipping cost is carefully considered.
  8. K4KYD

    K4KYD Ham Member QRZ Page

  9. G4COE

    G4COE Ham Member QRZ Page

    Ever seen computer speaker ratings they are usually over rated, just how do they measure wattage peak, RMS music power? I repaired a pair of computer speakers recently, 20Watt it said on the box, wow 20 Watts!!!!! The 2.5" speakers inside weren't even rated at 3 Watts - same with computer power supplies I would say.

    Before doing anything do yourself a favour put the supply on a load, measuring the power and ripple capability first, to get them going you have to ground the 'Power ON' wire..... the green wire on ATX supplies. I've heard of them self destructing if there isn't any load on the high current 5V supply, I can only guess they use the 5V as a reference for stabilizing the control switching, the regulating is done by varying the ON and OFF time of the chopper FET.

    It would be far better to build one, even a LM7812 or a LM317 with a pass transistor or two is far better than a computer power supply.... at least you will have no QRM from the switch-mode.


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